Renal transport and metabolism of low mol. wt. proteins, particularly peptide hormones such as growth hormone, parathyroid hormone and insulin, will be studied. The isolated perfused rat kidney will be used, an approach which permits the estimates of: (1) the concentration of proteins in glomerular filtrate, hence, filtered loads and absorption rates; (2) the metabolic fate of absorbed proteins; (3) renal pathways of extraction; (4) contribution of the kidney to the plasma turnover of proteins. Using isolated perfused tubules of rabbit kidneys, kinetics and mechanisms of tubular protein transport will be investigated with particular emphasis on specificity and cell activity of transfer mechanisms. The central aim is to elucidate the role of the kidney in the turnover and regulation of plasma levels of low mol. wt, proteins and the mechanisms by which this role is accomplished. An additional part of the proposal is the study of the influence of changes in protein concentration on proximal tubular fluid reabsorption in the isolated rat kidney preparation. Combined with perfusion on proximal tubules at known rates, this preparation offers the advantages of separating the influences of tubular load from peritubular factors since both sides of the epithelium can be altered experimentally. This technique is technically easier than the simultaneous perfusion of peritubular capillaries and tubules. The aim of the study is to make a quantitative assessment of the influence on oncotic pressure on proximal reabsorption.